The regulation of the cell cycle is governed and controlled by specific proteins, which are activated and deactivated mainly through phosphorylation/dephosphorylation processes in a precisely timed manner. The key proteins that coordinate the initiation, progression, and completion of cell-cycle program are cyclin dependent kinases (CDKs). Cyclin-dependent kinases belong to the serine-threonine protein kinase family. They are heterodimeric complexes composed of a catalytic kinase subunit and a regulatory cyclin subunit. CDK activity is controlled by association with their corresponding regulatory subunits (cyclins) and CDK inhibitor proteins (Cip & Kip proteins, INK4s), by their phosphorylation state, and by ubiquitin-mediated proteolytic degradation (see D. G. Johnson, C. L. Walker, Annu. Rev. Pharmacol. Toxicol 39 (1999) 295-312; D. O. Morgan, Annu. Rev. Cell Dev. Biol. 13 (1997) 261-291; C. J. Sherr, Science 274 (1996) 1672-1677; T. Shimamura et al., Bioorg. Med. Chem. Lett. 16 (2006) 3751-3754).
There are four CDKs that are significantly involved in cellular proliferation: CDK1, which predominantly regulates the transition from G2 to M phase, and CDK2, CDK4, and CDK6, which regulate the transition from G1 to S phase (Malumbres M, Barbacid M. Cell cycle, CDKs and cancer: a changing paradigm. Nat. Rev. Cancer 2009; 9(3):153-166). In early to mid G1 phase, when the cell is responsive to mitogenic stimuli, activation of CDK4-cyclin D and CDK6-cyclin D induces phosphorylation of the retinoblastoma protein (pRb). Phosphorylation of pRb releases the transcription factor E2F, which enters the nucleus to activate transcription of other cyclins which promote further progression of the cell cycle (see J. A. Diehl, Cancer Biol. Ther. 1 (2002) 226-231; C. J. Sherr, Cell 73 (1993) 1059-1065). CDK4 and CDK6 are closely related proteins with basically indistinguishable biochemical properties (see M. Malumbres, M. Barbacid, Trends Biochem. Sci. 30 (2005) 630-641).
A number of CDK 4/6 inhibitors have been identified, including specific pyrido[2,3-d]pyrimidines, 2-anilinopyrimidines, diaryl ureas, benzoyl-2,4-diaminothiazoles, indolo[6,7-a]pyrrolo[3,4-c]carbazoles, and oxindoles (see P. S. Sharma, R. Sharma, R. Tyagi, Curr. Cancer Drug Targets 8 (2008) 53-75). For example, WO 03/062236 identifies a series of 2-(pyridin-2-ylamino-pyrido[2,3]pyrimidin-7-ones for the treatment of Rb positive cancers that show selectivity for CDK4/6, including 6-acetyl-8-cyclopentyl-5-methyl-2-(5-piperazin-1-yl-pyridin-2-ylammino)-8H-pyrido-[2,3-d]-pyrimidin-7-one (PD0332991), which is currently being tested by Pfizer in late stage clinical trials as an anti-neoplastic agent against estrogen-positive, HER2-negative breast cancer. Tate, et al. describe the antitumor activity of the CDK4/6 inhibitor abemaciclib (LY2835219) (“Semi-Mechanistic Pharmacokinetic/Pharmacodynamic Modeling of the Antitumor Activity of LY2835219, a New Cyclin-Dependent Kinase 4/6 Inhibitor, in Mice Bearing Human Tumor Xenografts”, Clin Cancer Res (Jul. 15, 2014) 20; 3763-74). Rader, et al. describe the reduced proliferation in neuroblastoma-derived cell lines using the CDK4/6 inhibitor ribociclib (LEE011) (“Dual CDK4/CDK6 Inhibition Induces Cell Cycle Arrest and Senescence in Neorbalstoma”, Clin Cancer Res (Nov. 15, 2013) 19(22): 6173-82). VanderWel et al. describe an iodine-containing pyrido[2,3-d]pyrimidine-7-one (CKIA) as a potent and selective CDK4 inhibitor (see VanderWel et al., J. Med. Chem. 48 (2005) 2371-2387). WO 99/15500 filed by Glaxo Group Ltd discloses protein kinase and serine/threonine kinase inhibitors. WO 2010/020675 filed by Novartis AG describes pyrrolopyrimidine compounds as CDK inhibitors. WO 2011/101409 also filed by Novartis describes pyrrolopyrimidines with CDK 4/6 inhibitory activity. WO 2005/052147 filed by Novartis and WO 2006/074985 filed by Janssen Pharma disclose additional CDK4 inhibitors. WO 2012/061156 filed by Tavares and assigned to G1 Therapeutics describes CDK inhibitors. WO 2013/148748 filed by Francis Tavares and assigned to G1 Therapeutics describes Lactam Kinase Inhibitors. PCT Patent Application No. PCT/US2014/029073 filed by Strum et al. and assigned to G1 Therapeutics describes compounds and methods for protection of hematopoietic stem and progenitor cells against ionizing radiation using CDK4/6 inhibitors. PCT Patent Application No. PCT/US2014/028685 filed by Strum et al. and assigned to G1 Therapeutics describes compounds and methods for protection of normal cells during chemotherapy using CDK4/6 inhibitors. PCT Patent Application No. PCT/US2014/029429 filed by Strum et al. and assigned to G1 Therapeutics describes compounds and methods for treating Rb-positive cancers using CDK4/6 inhibitors. PCT Patent Application No. PCT/US2014/029274 filed by Strum et al. and assigned to G1 Therapeutics describes compounds and methods for treating certain cancers with CDK4/6 inhibitors.
It has recently been reported that the Pfizer CDK4/6 inhibitor palbociclib in combination with letrozole, while increasing progression free survival (PFS) compared with letrozole alone in post-menopausal women with estrogen receptor positive (ER+), human epidermal growth factor receptor 2 negative (HER2−) locally advanced or metastatic breast cancer, failed to statistically extend overall survival (OS), an important secondary endpoint. Based on the events at the time of the assessment, a median OS of 37.5 months was observed in the combination (paalbociclib+letrozole) arm versus 33.3 months for those who received letrozole alone, a difference of 4.2 months (HR=0.813, 95% CI: 0.492, 1.345). This OS observation at the time of final PFS analysis was not statistically significant.
Accordingly, there is an ongoing need for methods, combinations, and dosing regimens to treat patients with select Rb-positive cancers and abnormal cellular proliferative disorders.